In industry, electric drives use a large amount of electricity. Increasing the energy efficiency of these drives is therefore important both for environmental as well as for economic reasons. In addition to permanent magnet motors, there exist electric motors whose magnetic field is electrically excited. Such motors include separately excited DC motors, induction motors, separately excited synchronous motors, and synchronous reluctance motors. Excitation current causes energy losses for these types of motors. These losses can be classified as one of the following two types for common electric motor types: so-called core losses, and copper losses. The copper losses are caused by the current or the part of the current which is responsible for the excitation of the magnetic field. This part of the losses depends on the strength of the current and the winding resistance. Such winding losses also arise in windings from other materials, for example, aluminum. Since the winding resistance depends on the temperature, this part of the energy loss also depends on the temperature of the material. The other part of the loss arises in the magnetically soft material (for example, lamination steel) of the motor, when the induction changes.
An aspect of the present invention is to reduce the losses which arise from the electric excitement of the motor. To achieve this, field excitation is controlled so that the energy consumption of the machinery in which the drive is built in is reduced without any adverse impact on the dynamic properties of the drive with respect to the specific application.
Most highly dynamic electric drives work with so-called field oriented control (FOC). This control was originally developed for the dynamic control of three-phase induction motors, but can also be used for other kinds of electric motors. The basic idea of the field oriented control of a three-phase induction motor is that the stator current i1 can be separated into two components, i1d and i1q. Represented as space vectors, these two components are orthogonal to each other. The current component i1d is parallel with the space vector of rotor flux (Ψ2); i1q is perpendicular thereto. The magnitude of the rotor flux Ψ2 is determined by i1d. Torque m depends on the product of rotor flux Ψ2 and the current component i1q. Torque m obeys this product without any delay; in contrast, the rotor flux Ψ2 reacts to changes in the current component i1d with a large time lag. This time lag depends on the electric time constant of the rotor.
The maximum motor torque mmax is limited by the rotor flux Ψ2 and by the maximum of the current component i1q. The maximum motor current (stator current) that can be delivered by the inverter is limited by the inverter itself. The rotor flux Ψ2 on the other hand is limited by the core losses in the motor. At high excitation, the soft magnetic material of the motor goes into saturation and the hysteresis losses increase disproportionately. The flux is therefore most commonly not chosen over the nominal flux.
The time constant of the stator winding limits the maximal rate of change of the stator current. This implies that the inductance of the motor inhibits a jumpy increase of the current. The time constant of the stator winding is, however, at least an order of magnitude smaller than the time constant of the rotor. The time lag caused by the time constant of the stator winding can also be compensated by suitable current control in the inverter. A rapid increase of the motor torque is therefore limited by the incapability of rotor flux to be increased rapidly. The rotor flux of highly dynamic drives is therefore kept constantly large in order to make rapid changes in motor torque possible with minimum time lag.
Because of this, field oriented control systems work with a constantly high current component i1d in order to produce big rotor flux, even when a smaller flux would be equally sufficient for the actual operating point with a low torque requirement. Disadvantages of this method are the increased losses in the motor (core and winding losses), in the power switches of the inverter (switching losses and on state power dissipation), as well as losses in the passive components, such as in the power choke, the dc link capacitors, the wire connections etc.
A different control method is used to achieve higher speed. In this case, the motor flux is reduced in order to make higher speed than nominal possible in spite of limited stator voltage. This method is called field weakening. The reduced capability of the motor for creating torque must thereby be accepted.
High power drives which do not need high dynamics, for example, drives for railway vehicles, are often controlled differently. The current component i1d in such drives is not attempted to be keep constant, a constant ratio between i1d and i1q is much rather intended. This method is in particular often utilized in current source inverters (CSI). This control method is, however, not suitable for highly dynamic applications.